The present invention relates to video signal switching apparatus and circuitry, and in particular to electronic KVM switch circuitry and devices for switching very high speed video signals. The invention can be used to enable video signals from multiple computers with both digital and analogue video interfaces to be displayed.
In modern computing installations it is often useful to be able to use a single keyboard, video monitor and mouse to control several computers. By doing so the cost of additional monitors, keyboards and mice is avoided and a neater, more convenient installation that saves on power and space is provided.
Devices commonly known as KVM (Keyboard, Video monitor and Mouse) switches are used to allow KVM consoles to be shared between several computers. KVM switches are normally electronic devices because mechanical switches offer poor video quality and tend to wear out quickly. They are typically constructed using a microprocessor to signal the keyboard and mouse data and wide bandwidth buffer amplifiers or analogue switches to handle the video signals.
Prior art KVM switches have been designed to switch the analogue video interfaces that are commonly found on personal computers. These interfaces are typically implemented using a 15-way high density D-type connector that carries red, green and blue analogue colour signals and digital horizontal and vertical synchronisation signals. The bandwidth of these signals varies with the pixel resolution and screen refresh rate that the user chooses but a typical bandwidth would be around 100 MHz.
The analogue colour signals are designed to be transmitted along coaxial video cable with a characteristic impedance of 75 ohms. The quality of the video signal is maximised by terminating the video cable with its characteristic impedance in the monitor and ensuring that the coaxial cable acts as closely as possible to an ideal 75 ohm transmission line. However, KVM switches are connected in the video path between the computers and the monitor. The presence of the KVM switch effects the transmission line characteristics of the video path.
An acceptable video picture can be obtained using conventional buffer amplifiers or analogue switches to switch the video signals within the KVM switch. Given good design, the video quality obtained from such KVM switches can be made generally relatively tolerant to small imperfections in the transmission line characteristic of the video path from the computer to the monitor, as the signals are analogue and so the picture will be displayed even if there are imperfections. Minor imperfections are hard to detect with the human eye because they appear as slight colour changes and minor blurring. However, all KVM switches that handle analogue video will introduce some degree of degradation to the video picture. This degradation is more noticeable at higher screen resolutions and refresh rates. Also, in practice the video cables are not ideal transmission lines and so they also introduce a degree of picture degradation. This degradation gets worse as the cable length is increased.
While degradation of the video signal can be acceptable for low resolution analogue displays, it is not acceptable for high resolution displays. While digital video signals provide high fidelity, they are very high speed (ie high data signalling rate) and are consequently difficult to switch. If they are not handled correctly then no picture will be displayed at all. Therefore, there is a need for video signal switching circuitry which can switch digital video signals and provide high fidelity pictures.